Understanding Enteroliths in Horses

Updated January 24, 2025 | By: Andris J. Kaneps, DVM, PhD, DACVS, DACVSR

What are Enteroliths in Horses?

An enterolith is a smooth stone made of accumulated minerals which forms within a horse’s digestive tract. When a stone is large enough, it may obstruct the intestines and could lead to colic or gastrointestinal rupture.

Enteroliths are most commonly found in the large or small colon. There may be one or multiple present, and enteroliths can vary from being the size of a marble to a softball or larger. When enteroliths form in the gastrointestinal tract, the condition is known as enterolithiasis.

What Causes an Enterolith to Form?

Enteroliths form when certain minerals in your horse's digestive system combine under specific conditions. These minerals are present in your horse's diet through forage, water, and grain sources. Although generally safe, when these minerals are combined with an alkaline pH in the digestive tract under the right conditions, it could contribute to enterolith formation [1].

In many cases, the process begins when a horse ingests a foreign object, such as a small pebble, piece of metal, rubber band, or baling twine. These objects act as a core or focal point around which minerals start to accumulate. Over time, more layers of mineralization occur, forming a stone.

The most common mineral in enteroliths is struvite, a magnesium ammonium phosphate salt. Impurities like zinc, manganese, and iron may also be present. When an enterolith is cut open, it often reveals concentric layers of minerals, somewhat resembling the growth rings of a tree trunk.

Risk Factors for Developing Enteroliths

• Breed: Although they may occur in all horses, certain breeds tend to be associated with having enteroliths. Arabians and Arabian-crosses, Morgans, American Saddlebreds, Miniature horses, Quarter Horses, Thoroughbreds, and donkeys have been reported to have these stones [4, 6].
• Age: Enteroliths are highly uncommon in horses less than 2 years of age. The average age of affected horses in California was 11 years old [4], the median age of horses affected in Texas was 9-10 years old [5].
• Pasture Time: Horses that spend less than 50% of their time in turnout are more likely to have enteroliths than horses that have more turnout [5].
• Diet: Feeding more than 50% of the forage diet as alfalfa may increase the risk of enterolith formation [5]. Alfalfa hay is high in magnesium and protein - both are factors that may promote enterolith formation.
• Geography: Many cases are reported in the states of California, Arizona, and Florida, potentially due to the mineral composition of the soil, water, and forage sources. However, enteroliths may form in horses outside these areas.

How Common is Enterolithiasis?

Enterolithiasis is most commonly found in horses living in California and Florida but can occur in horses in many other regions as well [3].

A review of the medical records of 900 horses with enterolithiasis at the University of California Davis over a 23-year period provides information that’s helpful in understanding this disorder. It showed that horses with colic due to enteroliths represented 15% of patients admitted for treatment, and 28% of horses that required surgery for colic. Of the 900 horses, 131 (15%) had an intestinal rupture caused by the stones [4].

In a separate study conducted on horses presented to the Texas Veterinary Medical Center (TVMC) for colic, the frequency of enteroliths was much less common than in California. At TVMC, 26 horses were admitted due to enterolith obstructions during an 8-year period, which was only 1.7% of all colic admissions during that period [5].

Clinical Signs of Enteroliths in Your Horse

Depending on the size and number of intestinal stones, where they are located in the GI tract, and how much damage has been caused, signs may include:

• Long-term, recurring colic.
• Abdominal discomfort in horses may present as repeatedly lying down and getting up, rolling, pawing, looking back at the belly, sweating, and other signs of distress.
• Weight loss.
• Decreased appetite.
• Passing small stones in the manure.

Diagnosing Enteroliths in Horses

It can be difficult to make a diagnosis of enterolithiasis because the clinical signs are not very specific. If your horse continues to intermittently colic, then your veterinarian may perform a full abdominal work-up.

Radiographs of the abdomen are often part of a colic work-up in regions where enterolithiasis is common and in horses being evaluated for recurring colic. The x-rays typically help with diagnosing enterolithiasis, but the stones may not be visualized in all cases.

Treatment of Enterolithiasis

Surgery may be the only treatment option for horses recognized to have enteroliths that cause obstructions. The prognosis for full recovery following surgical removal of enteroliths is generally excellent. According to records from UC Davis, greater than 92% of horses that had surgery for enterolithiasis had a successful outcome in the long-term [4].

Strategies to Prevent Enteroliths in Horses

There are several management strategies that owners and care takers can implement in the barn to support normal digestion, including:

• Turn out your horse as often as possible to encourage normal movement of the feed through the digestive system.
• Try not to feed more than 50% of your horse’s forage as alfalfa.
• If situations require feeding more alfalfa, or your horse has other risk factors, talk to your veterinarian about administering apple cider vinegar. The apple cider vinegar may help decrease the pH, which acidifies the gut fluid, making the formation of enteroliths less likely.
• If your horse lives in a sandy environment, try to avoid having them eat from the ground and ingesting sand by using mats or elevated feed tubs. Also, speak with your veterinarian about occasionally supplementing your horse’s diet with psyllium to aid movement of feedstuffs through the intestines.
• Limit feeding bran, which contains higher levels of phosphorous.

Evidence-Based References

1. Hassel, D M et al. “Petrographic and geochemic evaluation of equine enteroliths.” American journal of veterinary research vol. 62,3 (2001): 350-8. doi:10.2460/ajvr.2001.62.350
2. Rouff, Ashaki A et al. “Trace elements in struvite equine enteroliths: Concentration, speciation and influence of diet.” Journal of trace elements in medicine and biology : organ of the Society for Minerals and Trace Elements (GMS) vol. 45 (2018): 23-30. doi:10.1016/j.jtemb.2017.09.019
3. Rakestraw, P. C., et al. "Equine surgery." (2012): 454-494.
4. Hassel, D M et al. “Evaluation of enterolithiasis in equids: 900 cases (1973-1996).” Journal of the American Veterinary Medical Association vol. 214,2 (1999): 233-7.
5. Cohen, N D et al. “Risk factors for enterolithiasis among horses in Texas.” Journal of the American Veterinary Medical Association vol. 216,11 (2000): 1787-94. doi:10.2460/javma.2000.216.1787
6. Albanese, Valeria et al. “Prevalence of Gastric Ulceration in Horses with Enterolithiasis Compared with Horses with Simple Large Intestinal Obstruction.” Veterinary sciences vol. 9,11 587. 25 Oct. 2022, doi:10.3390/vetsci9110587